1. Field of the Invention
The present invention relates to a Co-base magnetic alloy having excellent high-frequency magnetic properties, which is used in members of countermeasure against noise such as zero phase reactors and electro-magnetic shielding materials, inverter transformers, choke coils for active filters, antennas, smoothing choke coils, saturable reactors, power supplies for laser, pulse power magnetic members for accelerators, and so on. It also relates to high performance magnetic members made of the Co-base magnetic alloy.
2. Description of the Prior Art
Ferrite, amorphous alloys, nano-granular thin film materials, and so on have been known as magnetic materials for high frequency applications. The ferrite materials are unsuitable for high power applications in a high frequency range in which an operating magnetic flux density increases and a temperature rises, because the ferrite materials exhibit low saturation magnetic flux density and inferior temperature characteristics.
Because of large magnetostriction, Fe-base amorphous alloys have problems that magnetic properties are deteriorated under stress and that a large noise is generated in a use, wherein, for example, currents of an audio-frequency range are superimposed.
On the other hand, a Co-base amorphous alloy is thermally unstable. Therefore, if the Co-base amorphous alloy, which exhibits good properties for high-frequency applications, is used in applications which requires a high power, there will arise a problem that high-frequency magnetic properties are deteriorated because a large property change against time occurs.
An Fe-base nanocrystalline alloy is excellent in soft magnetic properties. Therefore, it is used for a magnetic core of common mode choke coils, high-frequency transformers, pulse transformers, etc. As typical alloy compositions thereof, there have been known an Fexe2x80x94Cuxe2x80x94(Nb, Ti, Zr, Hf, Mo, W, Ta)xe2x80x94Sixe2x80x94B alloy, Fexe2x80x94Cuxe2x80x94(Nb, Ti, Zr, Hf, Mo, W, Ta)xe2x80x94B alloy, and so on which are disclosed in JP-B2-4-4393 (corresponding to U.S. Pat. No. 4,881,989) and JP-A-242755. In general, these Fe-base nanocrystalline alloys are prepared by nanocrystalizing amorphous alloys by annealing, which are fabricated by quenching an alloy from a liquid phase or a gaseous phase. A single roll method, a twin roll method, a centrifugal quenching method, a method of rotary spinning in a liquid, an atomizing process, and a cavitation method are known as typical rapid quenching methods from the liquid phase. Further, known examples of rapid quenching methods from the gaseous phase include a sputtering method, a vapor deposition method, an ion plating method, and so on.
The Fe-base nanocrystalline alloy is prepared by nanocrystalizing the amorphous alloy prepared by the above methods by annealing, which is thermally stable not like as an amorphous alloy, and which has been known that it exhibits high saturation magnetic flux density which is substantially the same as those of the Fe-base amorphous alloy, and exhibits excellent soft magnetic properties and low magnetostriction. Further, it has been known that the nanocrystalline alloy exhibits a small property change against time and also excellent temperature characteristics.
When the Fe-base nanocrystalline soft magnetic alloy is compared with a conventional soft magnetic material having generally the same saturation magnetic flux density, the alloy exhibits higher magnetic permeability and lower magnetic core loss, so that it is excellent in soft magnetic properties. However, an optimum operating frequency range for use in the transformer is around several tens of kilohertz for thin strip materials, and the properties are not sufficient for applications in the high frequency. Moreover, when the alloy is used as members of counter-measure against noise, particularly a large effect is obtained at 1 MHz or less. Therefore, there has been a demand for materials superior in the property even in a higher frequency range. With regard to the members of countermeasure against noise for the high current, it is necessary to prevent the saturation of the magnetic core and the unstable operation. From this viewpoint, there has been a demand for a material which indicates a magnetization curve with a low squareness ratio and exhibits a superior property in a high-frequency range. In these uses, a high-permeability material having a relative magnetic permeability of several tens of thousands in a low-frequency region has a problem that the magnetic core material is magnetically saturated and that a sufficient property cannot be obtained in the high frequency range.
With regard to a magnetic switch for use in a saturable reactor, accelerator, and so on, there has been a demand for a magnetic core material which has a high squareness ratio and low magnetic core loss in order to improve controllability, compression ratio, and efficiency.
In order to solve the above problems, a thin film for reducing an eddy current loss, a high electric resistance granular thin film, and so on have been examined. However, the granular thin film with high electric resistance has a limitation in increasing a volume of the magnetic material, and it is difficult to use the thin film as the magnetic core material for a magnetic switch, transformer, choke coil, and so on in a pulse power applications handling a high energy and a large-capacity inverter.
Therefore, even for the thin strip material whose volume is easily increased, or a bulk material, there has been a strong demand for a material superior in the magnetic property in a higher frequency range as the magnetic core material.
The Fe-base nanocrystalline soft magnetic alloy manufactured by crystallizing an amorphous alloy thin strips by the heat treatment generally shows a high magnetic permeability in a frequency range of several hundreds of kilohertz or less, and exhibits a rather high value of a quality factor Q as one of important properties of the material for coil members. However, a sufficiently high Q cannot be obtained in a megahertz (MHz) or higher range, even when the alloy is heat-treated in a magnetic field and a magnetic anisotropy is induced in the alloy. Moreover, there are problems of a magnetic saturation of the material by superimposed direct-current or by an unbalanced signal, when the material is used in the choke coil for a three-phase power line.
As the Co-base nanocrystalline alloy, an alloy disclosed in JP-A-3-249151 (corresponding to U.S. Pat. No 5,151,137) is known. However, the disclosed alloy contains a large amount of borides. There are problems that even with the heat treatment in the magnetic field, a high Q in the high frequency range, and a sufficiently low squareness ratio, or a sufficiently high squareness ratio cannot be obtained.
To solve the above problems, as a result of intensive studies, the present inventors have found a Co-base magnetic alloy which has excellent high-frequency magnetic properties in the megahertz (MHz) range.
The Co-base magnetic alloy has a chemical composition represented by the following general formula, by atomic %: (Co1-aFea)100-y-cMxe2x80x2yXxe2x80x2c, where Mxe2x80x2 is at least one element selected from the group consisting of V, Ti, Zr, Nb, Mo, Hf, Sc, Ta and W; Xxe2x80x2 is at least one element selected from the group consisting of Si and B; and a, y and c satisfy the formulas of a less than 0.35, 1.5xe2x89xa6yxe2x89xa615, and 4xe2x89xa6cxe2x89xa630, respectively. At least a part of the alloy structure of the Co-base magnetic alloy consists of crystal grains having an average grain size of not more than 50 nm. The present invention is based on finding that the above Co-base magnetic alloy, having a relative initial permeability of not more than 2000, exhibits excellent high frequency magnetic characteristics in the megahertz (MHz) range.
The Co-base magnetic alloy is prepared by quenching a molten metal having the above chemical composition by means of a rapid quenching technique such as a single roll method to produce an amorphous alloy. The amorphous alloy is subjected to working and heat treatment at a crystallization temperature or a higher temperature to form fine crystal grains having an average grain size of not more than 50 nm. The amorphous alloy prior to the heat treatment preferably has no crystalline phase, but may partially include the crystalline phase. The heat treatment is usually performed in inert gases such as an argon gas, nitrogen gas, or helium gas, and so on. A magnetic field having an intensity enough for saturating the alloy is applied during at least a part of a heat treatment period, the heat treatment is performed in the magnetic field, and a magnetic anisotropy is induced. The magnetic field strength depends on a shape of a magnetic alloy core. However, in general, when the magnetic field is applied in a width direction of a thin strip (in a height direction of a wound magnetic core), a magnetic field of 8 kA/m or more is applied. When the heat treatment is performed under magnetic field applied along a magnetic path direction, a magnetic field of about 8 A/m or more is applied. Any one of a direct-current, alternating-current, and repeated pulse magnetic fields may be used as the applied magnetic field. The magnetic field is applied in a temperature range of 300xc2x0 C. or more usually for 20 minutes or more. When a magnetic field is applied during heating, at a constant temperature, and during cooling, the quality factor Q in the high frequency range, or a squareness ratio is improved, whereby a satisfactory result is obtained. On the other hand, when the heat treatment is performed without magnetic field, that is, when the heat treatment in the magnetic field is not applied, the high-frequency magnetic property is deteriorated. The heat treatment is preferably performed in the inert gas atmosphere whose dew point is usually xe2x88x9230xc2x0 C. or less. When the heat treatment is performed in the inert gas atmosphere having a dew point of xe2x88x9260xc2x0 C. or less, a variance of properties is small and a more satisfactory result is obtained. A maximum reaching temperature during the heat treatment is equal to or higher than a crystallization temperature, and is usually in a range of 450xc2x0 C. to 700xc2x0 C. In the case of a heat treatment pattern for keeping the alloy at a constant temperature, a keeping time at the constant temperature is usually not longer than 24 hours, preferably not longer than 4 hours, from the viewpoint of productivity. An average heating rate during the heat treatment is preferably 0.1xc2x0 C./min to 200xc2x0 C./min, more preferably 0.1xc2x0 C./min to 100xc2x0 C./min, an average cooling rate is preferably 0.1xc2x0 C./min to 3000xc2x0 C./min, more preferably 0.1xc2x0 C./min to 100xc2x0 C./min, and an alloy superior particularly in the high-frequency magnetic property is obtained in this range. The heat treatment is not limited to one step, and multi-step heat treatment or a plurality of heat treatments can also be performed. Furthermore, when a direct-current, alternating-current or pulse current is passed through the alloy, the alloy is allowed to generate heat and can also be heat-treated.
According to the above-described process, it is easy to provide the invention alloy with a relative initial permeability of not more than 2000. It is also possible for the invention alloy to have properties of not less than 4 of the quality factor Q at 1 MHZ, and a squareness ratio Br/B8000 of 20% or less. According to another embodiment of the invention, it is easily possible to provide the invention alloy with a squareness ratio Br/B8000 of not less than 85% by changing the orientation of magnetic field applied to the thin strip during heat treatment from the width direction to a longitudinal direction of the thin strip. Here, B8000 denotes a magnetic flux density with application of a magnetic field of 8000 Amxe2x88x921. Particularly, in the case of the a relative initial permeability of not more than 1000, the quality factor Q becomes particularly high, so that a good result can be obtained.
In the present invention, an Fe content ratio needs to be a less than 0.35. When a is 0.35 or more, a sufficient induced magnetic anisotropy cannot be obtained. When a magnetic field sufficient for saturating the alloy is applied in a direction substantially perpendicular to a magnetization direction during use and the heat treatment is performed, a considerable decrease of Q in 1 MHz occurs. Moreover, when the magnetic field sufficient for saturating the alloy is applied in generally the same direction as the magnetization direction during use and the heat treatment is performed, and when a is 0.35 or more, the squareness ratio is liable to drop unfavorably. A particularly preferable range is a less than 0.2. In this range, a magnetostriction is small, a high Q or a high squareness ratio is obtained, property deterioration due to stress is reduced, so that more preferable results can be obtained. The elements Mxe2x80x2 and Xxe2x80x2 promote amorphous formation. The element Mxe2x80x2 is at least one element selected from V, Ti, Zr, Nb, Mo, Hf, Sc, Ta and W, an Mxe2x80x2 amount y is in a range of 1.5xe2x89xa6yxe2x89xa615, and an Xxe2x80x2 amount c is in a range of 4xe2x89xa6cxe2x89xa630. When y is less than 1.5 atomic %, a fine crystal grain structure is not obtained after the heat treatment, and unfavorably a high Q is not obtained. When y exceeds 15 atomic %, the temperature property is disadvantageously deteriorated. The element Xxe2x80x2 is at least one element selected from Si and B. When the Xxe2x80x2 amount c is less than 4 atomic %, the crystal grains after the heat treatment is not easily finely divided. When c exceeds 30 atomic %, the saturation magnetic flux density disadvantageously decreases. Particularly, when a B (boron) content is from 4 to 15 atomic %, the induced magnetic anisotropy increases and an excellent property of a high Q or a high squareness ratio can be obtained.
A remaining part of the crystal grains having the average grain size of not more than 50 nm is mainly an amorphous phase. With a larger ratio of crystal grains, the induced magnetic anisotropy increases, and the quality factor Q at a higher-frequency is improved. However, the amorphous phase, which is partially present, realizes a high resistivity, the ultra-fine crystal grains, a good soft magnetic property, whereby a satisfactory result can be obtained.
With regard to the invention alloy, if necessary, the surface of the alloy thin strip is coated with particles or films of SiO2, MgO, Al2O3, and so on, the surface is treated by a formation treatment, an oxide layer is formed on the surface by an anode oxidation treatment, and an interlayer insulation treatment is performed. Then, a more satisfactory result is obtained. This particularly reduces an influence of an eddy current in a high frequency extending among the layers, and effectively improves the properties such as Q in the high frequency range and magnetic core loss. This effect is remarkable, when the alloy is used for a magnetic core made of a thin strip having a satisfactory surface state and a broad width. Furthermore, when the magnetic core is prepared using the invention alloy, impregnation, coating, and so on can also be performed as the occasion demands. The invention alloy can fulfill capabilities most for use in the high frequency range, but can also be used in a sensor or a low-frequency magnetic member. Particularly, the alloy can fulfill superior properties, when the member is easily magnetically saturated.
For the invention alloy subjected to the heat treatment while the magnetic field is applied in the direction substantially perpendicular to the magnetization direction during the use, the high Q is obtained in the high frequency even with the thin strip, as compared with a conventional thin strip material. Moreover, the superior properties can similarly be obtained even with the thin film or the powder. The quality factor Q is represented by a ratio of a real part xcexcxe2x80x2 of the magnetic permeability to an imaginary part xcexcxe2x80x3 of the magnetic permeability. The factor is one of the properties indicating the capabilities of the magnetic core material in the high frequency. When the material having a higher Q is used in the coil member, the loss is reduced and the properties are improved.
A static B-H loop of a hard magnetization axis direction of the Co-base magnetic alloy according to the present invention has a flat inclined shape, and usually has an anisotropic magnetic field HK of 950 Amxe2x88x921 or more. Even when a large magnetic field is applied to the present alloy, the material is not easily magnetically saturated, and the alloy is suitable for use in the high power. The relative initial permeability is about not more than 2000, and decreases little and exhibits a flat frequency dependence even in a high frequency range, as compared with a conventional nanocrystalline alloy thin strip having the same strip thickness.
In the present invention, 10 atomic % or less of a total amount of Co and Fe may be replaced with at least one element selected from the group of Cu and Au. With the replacement with Cu, Au, the crystal grains are more finely divided, and the high-frequency magnetic property is further improved. A particularly preferable replacement amount is 0.1xe2x89xa6xxe2x89xa63 (atomic %). In this range, the alloy can easily be manufactured, and particularly superior high-frequency magnetic properties such as the high Q can be obtained.
In the invention alloy, Co may be partially replaced with Ni, whereby it is possible to improve the corrosion resistance of the alloy and adjust the induced magnetic anisotropy of the alloy.
Moreover, in the invention alloy, Mxe2x80x2 may partially be replaced with at least one element selected from Cr, Mn, Sn, Zn, In, Ag, platinum group elements, Mg, Ca, Sr, Y, rare earth elements, N, O and S. Since Mxe2x80x2 is partially replaced with at least one element selected from Cr, Mn, Sn, Zn, In, platinum group elements, Mg, Ca, Sr, Y, rare earth elements, N, O and S, effects such as improvement of the corrosion resistance, enhancement of the resistivity, and adjustment of the magnetic property can be obtained. Particularly, the platinum group elements such as Pd and Pt can enhance the induced magnetic anisotropy, and can improve the properties such as Q in the higher-frequency range.
Moreover, Xxe2x80x2 may partially be replaced with at least one element selected from C, Ge, Ga, Al and P. By such a replacement, effects such as adjusted magnetostriction and fine crystal grains can be obtained.
A part of the invention alloy is of a structure of crystal grains having an average grain size of not more than 50 nm. A ratio of the crystal grains in the alloy structure is preferably 30% or more, more preferably 50% or more, particularly preferably 60% or more. A particularly preferable average crystalline grain size is in a range of 2 nm to 30 nm. In this range, a particularly high Q is obtained in a high frequency of 1 MHz or more.
The above mentioned crystal grains formed in the invention alloy are mainly of a crystalline phase primarily containing Co, in which Si, B, Al, Ge, Zr, etc. may be also dissolved. The crystalline phase may also contain an ordered lattice. The residual part other than the crystalline phase is mainly an amorphous phase. An alloy consisting essentially of only the crystalline phase may be also included in the present invention. With the alloy containing Cu or Au, a face-centered cubic structure phase (fcc phase) partially including Cu or Au may be sometimes present.
Moreover, when the amorphous phase is present around the crystal grains, the resistivity increases. By suppression of crystalline grain growth, the crystal grains are finely divided, the soft magnetic properties are improved, and therefore a more satisfactory result is obtained.
When a compound phase is not present in the invention alloy, more superior high-frequency magnetic properties are obtained.
Further, in the invention alloy, when at least a part or all of the crystal grains having an average grain size of not more than 50 nm are crystal grains having a body-centered cubic structure (bcc), the induced magnetic anisotropy is increases and a particularly superior high-frequency magnetic properties are obtained. In the invention alloy, at least a part or all of the crystal grains having an average grain size of not more than 50 nm may be crystal grains having a face-centered cubic structure (fcc), and superior soft magnetic properties and low magnetostriction are obtained. In the invention alloy, at least a part or all of the crystal grains having an average grain size of not more than 50 nm may include hexagonal (hcp) crystal grains.
According to another aspect of the present invention, there is provided magnetic members consisting of the above Co-base magnetic alloy. The wound magnetic cores or laminated magnetic cores made of the invention alloy with a conductive wire realize high performance transformers, choke coils or inductors, which exhibit a high Q in the high frequency range. The invention alloy is suitable for members of countermeasure against noise, since a sheet made of the invention alloy exhibits a high Q in the high-frequency range. When the alloy is used as cores for tuning type high-frequency accelerators, they exhibit superior properties. A magnetic members made of the Co-base magnetic alloy having a high squareness ratio can realize the superior properties as a magnetic switch core, etc.